1. Field of the Invention
The present invention relates to a positioning device, and more particularly to such a device included in a lithographic projection apparatus.
2. Description of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The projection system may include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a lens. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such multiple stage devices, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, which is commonly referred to as a step-and-scan apparatus, each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Lithographic apparatuses may employ various types of projection radiation, such as ultra-violet light (UV), extreme UV, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc. The apparatus may comprise components that are operated in vacuum, and are correspondingly vacuum-compatible. As mentioned above, the apparatus may have more than one substrate table and/or mask table.
A positioning device using Lorentz-motors is known from EP-B-0 342 639. The first part of the known positioning device comprises a slide that, by means of a system of servomotors, can be displaced with relatively low accuracy over relatively large distances in the X- and Y-directions. The second part comprises a substrate holder that, by means of a system of Lorentz-force motors, can be displaced relative to the first part with relatively high accuracy over relatively small distances with six mutually independent degrees of freedom. By using the system of Lorentz-force motors, mechanical contacts between the first part and the second part, and transmission of mechanical vibrations from the first part to the second part, are limited. This results in a high positioning accuracy of the known positioning device. An embodiment of the known positioning device comprises a further system of permanent magnets which exerts a supporting force on the second part in a direction parallel to the vertical Z-direction. This avoids the need to support the second part in the vertical Z-direction by Lorentz forces generated by the system of Lorentz-force motors, which would lead to a high energy dissipation in the coils of the Lorentz-force motors. Consequently, the Lorentz-force motors are used only to generate displacements of the second part, so that the energy dissipation of the electric coils of the Lorentz-force motors is considerably reduced.
A drawback of the known positioning device is that the magnitude of the supporting force of the further system of permanent magnets changes substantially if the distance between the first part and the second part changes in the Z-direction, so that the further system of permanent magnets has a substantial magnetic stiffness in the Z-direction. As a result of this stiffness, a mass spring system formed by the further system of permanent magnets and the displaceable mass of the second part exhibits a rather high natural frequency in the Z-direction, resulting in a substantial transmission of mechanical vibrations from the first part to the second part. This adversely affects the positioning accuracy of the known positioning device.
It is an object of the invention to provide a positioning device in which the above-described drawback of the known positioning device is alleviated.
The present invention relates to a positioning device comprising a first part and a second part that is displaceable relative to the first part by means of a system of Lorentz-force motors. More particularly, the invention relates to such a device comprised in a lithographic projection apparatus comprising a radiation system for supplying a projection beam of radiation; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
According to the present invention, there is provided: a positioning device comprising a first part and a second part which is displaceable relative to the first part by means of a system of Lorentz-force motors, characterized in that, relative to the first part, the second part is supported in a Z-direction by means of at least one gas cylinder, said gas cylinder comprising a housing having a pressure chamber, which housing is coupled to the first part; and a piston which is coupled to the second part and which can be displaced in the pressure chamber in the Z-direction, said piston being journaled relative to said housing at right angles to the Z-direction.
A gas cylinder as here referred to is sometimes also referred to as a (frictionless) pneumatic cylinder. By using the gas cylinder in the manner described above, the second part is supported relative to the first part by a pneumatic supporting force that is determined by gas pressure present in the pressure chamber. By a suitable choice of supply pressure and area of the piston, it is achieved that the pneumatic supporting force of the gas cylinder remains substantially constant when the piston is moved in the Z-direction. Since the piston is journaled perpendicularly to the Z-direction relative to the housing of the gas cylinder, e.g. by means of a static gas bearing, the piston can be displaced in the Z-direction substantially without friction, so that the relatively constant supporting force of the gas cylinder is substantially uninfluenced by the frictional forces of the static gas bearing exerted on the piston.
In the invention, the gas cylinder, acting as a gravity compensator, functions by providing compressed gas (e.g. air) which acts upon a cross section of a moving piston with a fixed projected area in the vertical (or other intended) direction. This area can be provided by a single physical surface, but can also be distributed over a number of physical surfaces, or even be a differential area between two opposing surfaces. The counterbalance force provided by the pressure acting on this area should remain as near constant as possible, both in magnitude and direction, irrespective of horizontal, vertical, pitch, yaw or roll motion of the supported part (e.g. mask or substrate holder), and its point of application should also remain static relative to the supportive part.
In a particular embodiment of a positioning device in accordance with the invention, the second part is supported relative to the piston in the Z-direction by a further static gas bearing, by means of which the second part is guided over a supporting surface of the piston so as to be displaceable at right angles to the Z-direction. By using the further static gas bearing, the second part can be displaced perpendicularly to the Z-direction substantially without friction over the supporting surface of the piston.
In another embodiment of a positioning device in accordance with the invention, the positioning device is provided with an intermediate part which is secured by means of an elastically deformable connecting member to the second part and bears on the supporting surface of the piston via the said further static gas bearing, the connecting member being substantially undeformable in the Z-direction but bendable about two mutually perpendicular bending axes perpendicular to the Z-direction. By using the intermediate part, the second part has freedom of rotation, relative to the first part, about the first axis of rotation and the second axis of rotation.
Optionally in accordance with the invention, the positioning device comprises an intermediate part that is provided with the supporting surface and is supported relative to the piston by means of a (substantially) spherical, static gas bearing in the Z-direction. By using the intermediate part and the spherical static gas bearing, the second part has freedom of rotation, relative to the first part, about the first and the second axes of rotation. The frictionless nature of the spherical gas bearing allows the second part to pitch and roll freely without influence from the gas cylinder.
Also optionally in accordance with the invention, the positioning device is provided with three gas cylinders and three Z-Lorentz-force motors belonging to a system of Lorentz-force motors, each of the Z-Lorentz-force motors exerting, in operation, a substantially dynamic Lorentz force on the second part in the Z-direction, in parallel with the substantially static force provided by the gas cylinder. The three gas cylinders provide, in the Z-direction, a stable and statically determined support of the second part relative to the first part, e.g. against gravitational acceleration. By means of the three Z-Lorentz-force motors, the second part can be displaced relative to the first part in the Z-direction, and rotated about the first axis of rotation and the second axis of rotation. Since each of the gas cylinders can be incorporated as part of the Z-Lorentz-force motor, a practical and compact construction of the positioning device is obtained.
In yet another embodiment of a positioning device in accordance with the invention, the first part can be displaced relative to a base of the positioning device, at least in the X-direction, by means of a drive unit of the positioning device. In this embodiment, the first part can be displaced relative to the base of the positioning device over relatively large distances with relatively low accuracy by means of said drive unit, while the second part can be displaced with relatively high accuracy over relatively small distances relative to the first part by means of the system of Lorentz-force motors. As a result, the drive unit, which must have relatively large dimensions, may be of a relatively simple type with a relatively small positioning accuracy, while the dimensions of the relatively accurate Lorentz-force motors can be limited.
In a lithographic apparatus according to the invention, at least one of the object tables includes a positioning device as described above, the substrate or mask holder being secured to the second part of the positioning device. The favorable properties of the positioning device in accordance with the invention manifest themselves in a particular way in the lithographic device in accordance with the invention in that transmission of mechanical vibrations to the substrate or mask holder is precluded as much as possible. This has a favorable effect on the accuracy with which the substrate or mask holder can be positioned relative to the projection system, and on the accuracy with which the pattern or sub-pattern on the mask is imaged onto the substrate.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising a radiation system for supplying a projection beam of radiation; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with substrate holder for holding a substrate; and a projection system for imaging irradiated portions of the mask on to target portions of a substrate; the method comprising the steps of: providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a mask that contains a pattern; projecting an image of at least part of the mask pattern onto a target area of the layer of radiation sensitive material; characterized in that, during said step of projecting an image, at least one of the mask and the substrate is positioned using a positioning device according to any one of claims 1 to 10.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cprojection beamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation, EUV, X-rays, electrons and ions.